1. Field of the Invention
This invention relates to a state of charge calculation device and a state of charge calculation method
2. Description of the Related Art
A secondary battery is used as a main or auxiliary power source in various fields such as automobiles, hybrid vehicles and electric carts. In order to operate this secondary battery efficiently, it is necessary to accurately execute charge and discharge control. Accordingly, a method for accurately calculating the state of charge (hereinafter referred to as “SOC”) of the secondary battery is required.
A method which uses current integration of charge and discharge currents has been known as a method for calculating the SOC of a secondary battery. However, with this current integration method, it is sometimes problematic to calculate the SOC of the secondary battery with a high degree of accuracy, when a battery is used that has not been fully charged or fully discharged. This is because, accuracy is strongly influenced, for example, by errors resulting from the omission of LSB digits of the CPU for calculating the SOC, or a fall in charge due to self-discharge. Furthermore, accuracy is also highly dependent on the accuracy of the current sensor.
As a result, a method in which an SOC estimate is corrected using an estimated voltage of a battery is proposed in PCT Laid-Open Publication No. WO99/61929.
However, with this SOC estimate correction method using estimated voltage, the accuracy of the SOC estimate using voltage can fall, depending on the SOC level. As a result, the accuracy of the calculation of the SOC of the battery is dependent on the charge and discharge pattern of the battery. Accordingly, the accuracy of the calculation the SOC of the battery sometimes deteriorates depending on the charge and discharge pattern. In addition, charge and discharge levels which cause the accuracy of the SOC estimate to deteriorate, are mainly within the normal usage range of the battery.
Moreover, in Japanese Patent Laid-Open No. 11-174134, a device is proposed for correcting the SOC estimate when the SOC value is within a predetermined range.
The device disclosed in this publication includes an SOC detection circuit for detecting the SOC of the secondary battery, a region specification circuit for specifying the current-voltage regions, a confirmation circuit that confirms whether sufficient charging and discharging to depart from a specified region has been executed, and a prohibition circuit which prohibits detection operations by the SOC detection circuit until confirmation by the confirmation circuit is completed. In addition, if the calculated SOC is below a lower limit value (for example, 20 percent) or above an upper limit value (for example, 80%), SOC correction is executed. Furthermore, SOC correction is not resumed until the corrected SOC has departed from a predetermined region. As a result, errors in the SOC estimate are prevented.
Accordingly, when the secondary battery is being used normally, namely, when the secondary battery is being repeatedly charged and discharged, a significant difference between the calculated SOC (hereinafter referred to as the “calculated value”) and the actual SOC (hereinafter referred to as the “actual value”) does not occur.
However, if the secondary battery naturally discharges, the SOC reduces. As a result, the actual value becomes significantly different from the calculated value. For example, the SOC of a secondary battery of an automobile that has not been used for a long time reduces due to natural discharge. However, the calculated value indicates the last calculated SOC. At such a time, the calculated value is higher than the actual value. Moreover, if the secondary battery is replaced, a secondary battery is used which is different from the secondary battery for which SOC was last calculated. As a result, the calculated value and the actual value are substantially different.
In the case that the secondary battery naturally discharges, the calculation device disclosed in the aforementioned publication executes calculation of the SOC. The SOC value of the secondary battery which is displayed on a monitor or the like, reduces until the difference between the calculated value and the actual value is eliminated. However, the SOC display continues to show the reduction in the SOC even when the secondary battery is charged. FIG. 8 shows changes in the calculated value and the actual value when the device disclosed in the aforementioned publication executes calculation of the SOC.